1. Field of Invention
This invention relates to an electronic transmitter for transmitting pressure values of fluids in industrial processes.
2. Description of the Prior Art
Transmitters used for transmitting pressure values and, more specifically, values of pressure changes occurring in industrial processes of any suitable type, are already known. For example, well known in the art are the transmitters of the capacitive type.
A relevant problem not yet satisfactorily solved hitherto in these transmitters is that relating to the provision of means sensitive to the process fluid pressure, more particularly, the measuring diaphragm subjected to the pressure changes. There are two types of diaphragms, namely a convolute diaphragm which is difficult to manufacture and is suitable for large displacement and has a relatively low sensitivity, as well as a flat, unshaped measuring diaphragm which enables short displacements to be performed and has a good sensitivity; this second type of diaphragm must be machined to a maximum of precision and must be mounted in a very accurate manner in the instrument in order to attain the desired results.
The convolute diaphragm has good linearity characteristics even when large displacements are involved. It is usually employed in sensors requiring large displacements of the measuring element (on the order of millimeters) and which do not need to be machined to a high grade of finishing. On the other hand, this diaphragm is very difficult to manufacture and its construction causes some difficulties (reproducibility of both the shape and the elastic characteristics of the diaphragm is difficult).
The flat, unshaped diaphragm has, in general, a bad linearity characteristic for large displacements; this characteristic will, however, be gradually improved if the diaphragm is subjected to small or very small displacements. In this case, however, a very sensitive and accurate sensor has to be used. On the other hand, the flat, unshaped diaphragm has the outstanding advantage of ensuring repeatability of the production operations and, accordingly, of an improved reproducibility and consistency of both the characteristics and the performances of the diaphragm.
A further problem encountered when dealing with the discussed type of transmitters is the protection of the measuring diaphragm against overpressures, the so-called "ever-range".
Also to be taken into consideration is the general structure of the apparatus which has to meet many different requirements at the same time, such as compactness, strength, environmental protection and so on.
Electronic transmitters of pressure values of industrial process fluids are also known, which include an inductive sensor comprising a housing formed of a pair of half-housings defining therebetween an intermediate chamber in which a measuring diaphragm is fitted, which is locked between the two half-housings and directly or indirectly subjected to the process fluid pressures on one or both sides of the measuring diaphragm and having on at least one side of the measuring diaphragm magnetic cores provided with coils each of which is adapted to induce a magnetic field whose inductance changes caused by the gap changes due to the measuring diaphragm movement are processed in an electronic circuit for evaluating the pressure value acting at that time on the measuring diaphragm.
However, these devices, while giving good performances, have disadvantages such as an incomplete linearity response of the measuring diaphragm, a somewhat weak output signal requiring rather complex processing electronics and incomplete safety against process fluid over-pressures which could damage the measuring diaphragm.